It is well-known to use naturally occurring and synthetic crystalline microporous materials as catalysts in various hydrocarbon conversion processes such as dehydrogenation, reforming, alkylation, etc. For example, the dehydrogenation of aliphatic hydrocarbons to the corresponding olefins can be carried out using as the catalyst a zeolite such as ZSM-5 or zeolite-L, on which has been deposited platinum metal.
Specifically, U.S. Pat. No. 4,990,710 discloses a tin containing microporous material on which is also deposited platinum. The material is used to catalyze dehydrogenation and dehydrocyclization of hydrocarbons. U.S. Pat. No. 4,104,320 discloses dehydrocyclizing aliphatic hydrocarbons in the presence of hydrogen and a catalyst consisting essentially of a L-zeolite having an exchangeable alkaline metal such as potassium and at least one metal from Group VIII of the periodic table and tin and germanium. U.S. Pat. No. 5,122,489 discloses a non-acidic dehydrogenation catalyst comprising a crystalline microporous material which contains a modifier selected from the group consisting of tin, thallium, indium, and lead and a dehydrogenation metal selected from the Group VIII metals of the periodic table. The microporous material has a Si/Al ratio of at least 2 and preferably at least 10.
U.S. Pat. No. 5,736,478 discloses a catalyst for dehydrogenating paraffins which comprises platinum, tin and potassium supported on an L-type zeolite. The catalyst is prepared by first impregnating a potassium L-zeolite with a Group IVA metal, e.g., tin, calcining and then impregnating with a Group VIII metal. U.S. Pat. No. 6,600,082 B2 discloses a PtSn catalyst in which the Sn is in a reduced state. The patentees state that the support can be zeolites such as ZSM-5, but the Sn is not in the zeolite framework. Finally, U.S. Pat. No. 5,518,708 discloses molecular sieves in which some of the aluminum atoms in the framework have been replaced by tin atoms.
The common characteristic of these dehydrogenation catalysts, is that both the dehydrogenation metal, e.g., platinum, and the modifier metal, e.g., tin, are either dispersed on the microporous material or are present “intrazeolitic”. By intrazeolitic is meant that the metals are in the channels of the microporous or zeolitic material. In contrast to these catalysts, applicants have developed a novel dehydrogenation catalyst in which the tin modifier metal is present as a tetrahedral oxide unit in the framework of the zeolite or microporous material. Specifically, the catalyst comprises a noble metal such as platinum dispersed on a non-acidic molecular sieve having a three-dimensional microporous framework structure of tin, aluminum and silicon tetrahedral oxide units and having an empirical formula on an anhydrous basis of mA:(SnwAlxSiy)O2 where A is at least one exchangeable cation, m is the mole fraction of A and w, x and y are the mole fractions of tin, aluminum and silicon respectively. Preferably the molecular sieve has the crystal structure of zeolite-L. Applicants' catalyst is also characterized in that at least a portion of the tin is in a reduced oxidation state and preferably in the zero valent state.